A fuel cell system of a hydrogen fuel cell includes a fuel cell stack for generating electrical energy from an electrochemical reaction between reaction gases (hydrogen and oxygen). A hydrogen supply device supplies hydrogen that is a fuel to the fuel cell stack, and an air supply device supplies air including oxygen that is an oxidizer to the fuel cell stack. A heat and water management system controls an operating temperature of the fuel cell stack and performs a water management function. A fuel cell system controller is configured to control all operations of the fuel cell system.
In general, the hydrogen supply device includes a hydrogen storage (hydrogen tank), a regulator, a hydrogen pressure control valve, a pressure sensor, a hydrogen recirculation device, and the like. The air supply device includes an air blower, a humidifier, and the like. The heat and water management system includes an electric water pump (cooling water pump), a water tank, a radiator, and the like.
The hydrogen tank of the hydrogen supply device supplies high-pressure hydrogen which is decompressed in the regulator and then supplied to the fuel cell stack. The amount of the decompressed hydrogen supplied to the fuel cell stack is controlled through a pressure control according to operating conditions of the fuel cell stack.
That is, the hydrogen passing through the regulator is pressure-controlled by the hydrogen pressure control valve and then supplied to the fuel cell stack. The hydrogen pressure control valve adjusts the pressure of the hydrogen decompressed by the regulator to be suitable for the operating conditions of the fuel cell stack. The supply amount of the hydrogen is controlled through hydrogen supply pressure control using the hydrogen pressure control valve.
The remaining hydrogen after the reaction in the fuel cell stack is discharged through an anode outlet of the fuel cell stack or recirculated through an anode inlet of the fuel cell stack by the hydrogen recirculation device.
The hydrogen recirculation device has various forms according to a recirculation method including a method using an ejector, a method using a recirculation blower, a method using together an ejector and a recirculation blower, and the like. The hydrogen recirculation device recirculates, to an anode of the fuel cell stack, the remaining non-reaction hydrogen after the hydrogen is used at the anode, thereby reusing the hydrogen.
In the fuel cell stack, as foreign substances such as nitrogen, water, and steam moved to the anode through an electrolytic membrane in the fuel cell stack increase, the amount of hydrogen in the anode decreases, and hence reaction efficiency of the fuel cell stack decreases. In this regard, a hydrogen purge valve is opened to purge the hydrogen for a predetermined period of time.
That is, the hydrogen purge valve for the hydrogen purging is installed between the anode outlet of the fuel cell stack to discharge the hydrogen of the anode periodically. Thus, the foreign substances such as nitrogen and water are discharged and removed together with the hydrogen, and efficiency of the fuel cell stack increases.
When the foreign substances in the fuel cell stack are discharged as described above, hydrogen concentration and utilization of hydrogen are increased, and gas diffusivity and reactivity are improved.
A fuel cell vehicle comprises an electric motor as a driving source for driving the vehicle and an inverter for converting a DC voltage of the fuel cell stack and a battery into an AC voltage to drive the electric motor.
FIG. 1 is a view illustrating a current sensor for sensing a stack current between a fuel cell stack and a system load according to the related art. Referring to FIG. 1, a current sensor 2 for sensing a current output from a fuel cell stack 1 is installed in a fuel cell system.
The current sensor 2 senses a current from the fuel cell stack 1 and sends to a system load 3 which includes an auxiliary machinery such as an air blower, an electric water pump, a recirculation blower or a radiator fan, an inverter, or the like. The stack current sensed by the current sensor 2 is input to a controller to be widely used in various system controls for a vehicle, including stack operation, auxiliary machinery operation, inverter (motor) driving control, and the like.
As described above, the current sensor is a main sensor for sensing a stack current (current consumed by a load) used as a variable in control of various systems of the vehicle including the fuel cell system. In a failure of the current sensor, a system malfunction occurs, and therefore, a logic for diagnosing a failure of the current sensor is applied to the vehicle.
When the failure of the current sensor is detected through a failure diagnosis logic, the fuel cell system immediately shuts down.
FIG. 2 is a view illustrating a process of shutting down a fuel cell system after a failure of a current sensor is diagnosed in a conventional art. A controller determines whether a pressure control of hydrogen supplied to a fuel cell stack is abnormal based on a sensing value of a pressure sensor during the operation of the fuel cell system (S1). The controller, according to a predetermined failure diagnosis logic, diagnoses a disconnection/short-circuit failure of the current sensor (S2), and diagnoses a performance failure of the current sensor (S3).
The controller immediately shuts down the fuel cell system in the determination of an abnormality (over-pressure/low pressure) of the pressure control of the hydrogen or a disconnection/short-circuit failure of the current sensor, or the determination of the performance failure of the current sensor, in which the sensing value of the current sensor is represented as a negative value (S4).
FIG. 3 is a view illustrating a failure diagnosis possible region and a failure diagnosis impossible region in an output of the current sensor according to the conventional art.
As shown in FIG. 3, a controller evaluates a current value by reading an output voltage of a current sensor. A disconnection region, in which a short circuit or a sensor performance failure is determined, exists in a sensing region of the current sensor, but a failure diagnosis impossible region also exists widely.
That is, a scaling failure having an error at a constant rate compared with an actual value, an offset failure having an error with a constant value compared with an actual value, a stuck failure where a sensing value of the current sensor is fixed to a certain value, or the like may exist even when the output of the current sensor is within a normal range. Although the current value, which has a deviation compared with the actual current value, is detected when such a failure occurs, it is impossible to sense the current value.
As a result, malfunction occurs in the various systems in the vehicle, including the fuel cell system, due to sensing a failure. Particularly, an error occurs in air supply control, cooling control, hydrogen purge and recirculation control, or the like, which depends on a sensing value of the current sensor.